This invention relates to a wavelength conversion element, and more particularly to a wavelength conversion element utilizing a non-linear optical material.
FIG. 2 shows a wavelength conversion element 100 equipped with a Fabry-Perot resonator for achieving high output realization of SHG (second-harmonic generation).
Such a conventional wavelength conversion element 100 is composed of a resonator 120 utilizing a so-called semiconductor laser of III-V group in the periodic table and a thin film 140 (or super lattice) made of a material such as ZnS/ZnSe of II-VI group in the periodic table which is attached to one longitudinal end of the resonator 120. Reflecting films 151,155 are disposed to the other longitudinal end of the resonator 120 and the front side of the thin film 140, and a waveguide path 125 formed of a material such as GaAlAs/GaAs of III-V group in the periodic table is formed in the resonator along the longitudinal direction thereof.
When an electric current is injected in the waveguide path 125 of the resonator 120 thus formed, near infrared rays are oscillated and the oscillated light is propagated in the waveguide path 125 and reaches the thin film 140 attached to the one end of the resonator 120. The semiconductor of II-VI group has the SHG activation, which carrys out wavelength conversion of the near infrared rays amplified by the resonator to thereby generate coherent light of blue color. In such an operation, it is necessary to select a thickness (or mutual operative length L1) of the thin film 140 so as to satisfy the resonating condition with respect to the oscillated light for the reason that a large wave-conversion efficiency is expected by satisfying a phase matching.
In view of the foregoing, in the conventional technology described above, the phase matching has been carried out by utilizing the II-VI group semiconductor thin film 140 having the mutual operative length L1 such as 600 nm being shorter than the coherent light length.
However, an output P.sub.2W of a second harmonic subjected to the wavelength conversion is expressed by a following function related to a non-linear optical constant .alpha., an output P.sub.W of a fundamental (or primary) light and the mutual operative length L. EQU P.sub.2W =.alpha..sup.2 P.sub.W.sup.2 L.sup.2 ( 1).
That is, the wavelength conversion efficiency is proportional to the squares of the non-linear optical constant .alpha., the output P.sub.W of a fundamental (or primary) light and the mutual operative length L, respectively. Accordingly, the conventional wavelength conversion element 100 of the type described above has a defect of not obtaining a remarkable wavelength conversion efficiency because the mutual operative length L is too short. Furthermore, the thin film 140 of the material of II-VI group is disposed to the longitudinal end of the III-V group semiconductor laser resonator, so that a process for producing a thin film 140 due to crystal growth is made complicated and a good yield is not expected, thus being inconvenient.